This invention relates to a novel, non-progressive trifocal ophthalmic lens comprising distance-vision, near-vision and intermediate-vision optic fields.
In the past, trifocal ophthalmic lenses have been used for vision correction. Trifocal lenses are used to correct for loss of accommodation, or loss of the ability to focus the eyes, on objects in both the near vision range and intermediate vision range as a result of a decrease in the elasticity of the lens of the eye.
Traditional trifocal ophthalmic lenses have been manufactured from glass or other transparent material that adjusts the focal point of an image of an object for the wearer of the lens. The image of the object is formed using a lens or other transparent material, usually having two polished surfaces to focus rays of light from the object. Both of the polished surfaces of a trifocal lens are usually curved with the front curve (the surface closer to the object) being convex and the back curve (the surface closer to the eye) typically being concave. The specific curvature of trifocal lens surfaces are typically made by grinding and lapping devices selected from a library with varying radii.
The lap devices operate by uniformly grinding out usually spherical or toric optic fields. After the surfaces are polished, the curved lens surfaces focus an image of an object by the process of refraction, in which rays of light bend where the rays enters the lens or leave the lens. With converging trifocal lenses, the rays of light are refracted and converge at a focal point. With traditional trifocal lenses, the polished surfaces can be spherical, i.e., the curved surfaces have a constant radius of curvature, or toric with different base and cross curves. A traditional measure of the power of a trifocal lens is measured in units called diopters, in which a diopter is equivalent to 1 divided by the desired focal length in meters.
Trifocal lenses have three sets of curved surfaces, each having a different radius of curvature. Progressive lenses are designed with a continuously increasing range of focal powers. However, progressive lenses produce several unwanted effects, namely distortion of images, aberrations and peripheral astigmatism. These several unwanted effects of progressive lenses cannot be eliminated. A disadvantage of traditional, trifocal ophthalmic lenses is that the transition zones between the different curved surfaces are discontinuous, i.e. there are sharp discontinuities in curvature between the curvatures of the three separate optic fields resulting in a line or curve that demarcates the different optic fields. As a result, the wearer of traditional trifocal lenses experiences a blurring effect when the wearer's eyes transit through the transition. Thus, these traditional trifocal lenses do not provide a clear, sharp, focused image when one views an object through one optic field and then another optic field, i.e. when one's eyes transit from an intermediate-vision optic field to a near-vision optic field. Moreover, these discontinuities in curvature provide an undesirable cosmetic effect for the wearer of traditional trifocal lenses.
With traditional trifocal lenses, these discontinuities in curvature between the distance-vision, near-vision and intermediate-vision optic fields are clearly undesirable properties. Moreover, the discontinuities in curvature between the three different optic fields are clearly apparent as visible outlines to an observer. All progressive lenses produce undesirable peripheral astigmatism and distortion aberrations for the wearer.
Accordingly, it would be desirable to provide a novel, non-progressive trifocal ophthalmic lens in which the transition zone between the optic fields is blended, i.e. to provide a trifocal lens that eliminates the lines associated with the discontinuities in curvature that characterize traditional trifocal lenses. Moreover, it would be desirable to provide a novel, non-progressive trifocal ophthalmic lens in which the transition zone between the optic fields is facilely formed during manufacture of lens molds.
The objective of a blended segment lens is simply to eliminate the line(s) present in traditional trifocal lenses. This is done by blending the two different curves of adjacent optic zones. The blend area should be controlled to make it as narrow as possible and consistent in width. There is no objective to provide vision in the blend zone.